Absorption modulation



June 5, 1944 R.` H. GEORGE ETAL 2,350,552y

ABS ORPTI ON MQDULATION Filed March 16, 1942 v4' 78 k.. M77

ATI'ORN EY :Patentedj'Jii-ne 6, 19de UNITED vSTATES PATENT OFFICE Ansonr'rloN MonULArioN e am n. George and melma c. webb, west Lafayette, Ind., assigner; to Radio Corporation ol' America, a corporation of Delaware Application March 1s, 1942, serial No. 434,194

4 claims. (ci. 17a-'1715) This invention relates to improvements in modulation systems and is concerned more particularly with a method and means by which the modulation of a high frequency carrier may be more readily\carried forward at low power and withl relatively small tubes. In its more specific form the invention is particularly directed to the modulation of a carrier frequency oi high value, such as that used at the present time in television'transmission orV the system is particularly adapted for the modulation of a carrier whose frequency is substantially within the general range of that now used in the transmission of television signals. Accordingly, the system herein to be described is particularly adaptable for use in the transmission of television signals where high detail is desired, and where the transmission can be carried forward with a mini-l mum expenditure of power.

According to the prior artvarlous methods have been suggested from' time to time for modulating the carrier frequency uponwhich the intelligence signal is to be transmitted. Among the various systems heretofore suggested and utilized to some extent at least were systems of the type known as the plate modulation systems. These systems have been used considerably for broadcasting and generally require high peakto-pealr modulating voltages, but for television transmissions it has been found that because the modulator tubes operate at relatively low plate emciency a plate modulation system of usual type is not particularly adaptable for use where a high power output is required.

In other forms of transmission the carrier 1 frequency energy has been modulated by the sca-called grid modulation types oi transmitters which do not require the same high modulating petentials in the tubes, but here again the emeiency is relatively low.

In still other forms of transmission the socalled absorption type of modulation has been used to some extent and in this type of system some desirable results have been obtained. One

' form of such a system is described, for instance, in the Proceedings of the Institute of Radio Engineers for August, 1933, in an article by W. N. Parker, commencing on page 946, (volume 26, No. 8), in which there is disclosed a method of absorption modulation upon which the present invention is to be considered as an improvement. The present invention has the distinct advantage over what has heretofore been known in the art because it provides a system where the two electron paths which perform the absorption' modulation are connected in parallel and in such a position across the radio frequency lines as to make' possible dropping the impedance to a vrelatively low value. At the same time the present invention always provides that the control electrodes of the modulator tubes are never driven positive, as in the prior art, so that grid current does not tend to flow in the system.

In one broad form the invention consists of providing an amplifier stage, preferably of the class C type, which is suitably coupledin A. C. or D. C. fashion to an electrical transmission line which is electrically equivalent in length to approximately one-quarter wave length of the carrier frequency to be radiated. 'Ihis line serves l essentially according to the present invention as the inal plate tank circuit of the final amplifier tube. Then connected cross such transmission line is an impedance matching section which is of the order of. one-quarter wave length long and which at its end point has a tapped connection of feeder lines' of any suitable or desired length to the antenna proper. AAt the same end point a further impedance matching section (which can be termed a modulator line) is connected. This section is approximately onequarter wave length long and the modulator tubes are connected to the end of this section in a manner which will hereinafter be explained.

It therefore becomes one of the primary objects of the invention to provide a system of absorption or impedance modulation which is relatively simple in its arrangement and construction, and which at the same time shall increase the emciency of operation over that heretofore known in the prior art systems. At the same time the present invention has as a further object that of providing a transmission system in which a more effective use of side bands in the modulation may be obtained and consequently substantial improvement in the emciency of transmisl sion.

Other objects and advantages of the present invention are those of increasing the fidelity of operation over the systems known in the prior art so that a more pleasing and a more accurate pictorial representation may be obtained where the invention is applied to television systems.

Other objects and advantagesV of the invention will become apparent from a consideration of the following specification taken in connection withvthe accompanying drawing, in which:

Figure 1 is a schematic representation of the transmissin line modulation system, and- Figure 2 is a diagrammatic representation of one form which the invention may assume in its installation.

Referring now to thev drawing, and first to Figure l thereof, the nal amplifier of lthe sys-- tem isl conventionally represented by the diagrammatic representation I. This amplier is preferably of the push-pull type and operating as a class C radio frequency amplier. The amplifier I is coupled to the line 3 either in A. C. or D. C. fashion. The line 3 is actually a loaded line, and while electrically of the order of onequarter the wave length of the radiated carrier, it vis actually slightly less than that.'amount. Positive voltage for the push-pull amplifier tubes of the amplifier I is supplied from the terminal point 5 so as to be connected to each of the two sections 2 and 4 of the line 3. The line3, including its component parts 2 andA 4, serves essentially as the nal plate of the tank circuit of the iinal amplifier tubes connected withinthe box Ill. Connected at the points 8 and 8 on the branches 2 and 4 ofthetransmisslon line 3 arev two additional conductors 1 and 9 which are of equal length and also about one-quarter wave length long. These conductors connect at their termination to the conductors A51 and 59 at junction points I4 and I5. The conductors 51 and 59 are arranged to receive energy from the lines 1 and 9 under the .control of the modulator tubes conventionally represented within the box II) and when so arranged this section of the line acts as an impedance inverter section. The line 1 and 9 preferably connects to thetransmission line 3 by way of smaller condensers II and I2 and at the junction points I4 and I5 there is provided an additional line I8 and I1 which leads to the antenna 2|. The section of the conductors 1 and 9 between the transmission line 3l and junction points I4 and I5 is also equal to onequarter wave length of the transmitted carrier, as is the section of the line 51 and 59 between the -junction points I 4 and I5 and the modulator I0, although the latter section is somewhat shortened physically because of the capacitive loading of the modulator tubes. The line section I8 and I1 which is connected at the points I4 the line 51-59, in which instance the sending end impedance would be across the junction points I4-I5 and the receiving end impedance would be represented by the modulator III.

Referring now to the schematic representation of the circuit diagram of Figure 2, the output electrodes of the final amplifier or the pushpull arrangement of Figure 1 are indicated by the elements 3| and 3l'. These are the plate or anode elements of the final amplifier of the video transmitter, and they are connected to the line type tank circuit 3, as hereinabove explained in connection with Figure l.

As was also pointed out in connection with "Figure `1, the platesupply for the tube plates 3l 'and 3|' is connected at the terminal 5, as was also shown by Figure 1, and is fed to the transmission line sections 2 and 4 through the high frequency choke 32 `and the adjustable connection to the transmission line provided by the slider element 33. The choke 32 may be bypassed to ground by the small condenserl 32'. The one-quarter wave-length transmission lines of low impedance, designated in Figure 1 by the lines 1 and 9, are similarly shown by Figure 2, as are the blocking condensers II and I2 connected to the terminal points 8 and 8. The transmission line section of Figure l represented between the points I4 and I5 and the modulator I0 is shown in the same relationship in Figure 2, and this line serves as an impedance inverting line and connects to the modulator tube 35 in a manner hereinafter to be explained.

With the impedance method of modulation, the arrangement herein disclosed is so arranged that the modulator tube 35 reflects a variable impedance which is preferably purely resistive against the antenna feeder junction points I4 and I5, which not only varies in voltage across The feeder lines 1 and 9 between the tank line and I5 to the lines 1 and 9 is not crictical as to its length but is made of approximately the same impedance as the line section 1 and 9. This line should also match closely with the input im' pedance to the antenna 2I. In this arrangement it becomes apparent that the lines 2 and 4 should be tuned to resonance with the conductors 1 and 9 removed therefrom. The points 6 and 8 at which condensers 1 and 9 connect to the lines 2 and 4 are established in such a manner that the flnal amplifier l is in resonance both while it is supplying power to the antenna, and

Aat times when a short circuit is placed at the junction points I4 and I5 on the lines 1 and 9.

Iffnow the schematic representation of Fig. ure l be further considered, it will be seen that the relationship of the product of the impedi ance at the sending end (that is, atpoints 3 and 8, for instance) and the impedance at the receiving end '(that is, the ends I4 and I5, for instance) of the transmission lines 1 and 9 shall be ZsZR=ZL2 where Zs is the sending end impedance, ZR is the receiving end impedance, and ZL is the line impedance. Under these circumstances, Zr. would be the impedance of the line 1-9 if the sending end impedance is across th'e junction points 6 and 8 and the receiving end impedance is across the junction points I4 and I5, or Z1. might represent the impedance of 3 and points I4 and I5 and lines I8 and I1 between the sarne points and the antenna 2| should preferably be of the same impedance for a video or television transmitter so as to eliminate any matching section.

In order to obtain the greatest percentage modulation with a given modulator tube, the lines 1 and 9 between the tank line 3 and points I4 and I5 and the lines 51 and 59 between the points I4 and I5 and the modulator tube 35, as

well as the lines I8 and I1 between the antenna and the connecting points I4 and l5 must be designed to give the most satisfactory range of ilripedance change at the junction points I4 and In the prior art arrangements heretofore 1%- ferred to, provision was made whereby an even number of modulator tubes might be so arranged that their plates connected to the lines 51 and 59 and their filaments connected to ground. However, the arrangement then provided was such that only half of the radio frequency voltage is impressed across each tube, so that the equivalent mean resistance of the modulator can be substantially lreduced by the present invention through the connection of the modulator tube sections directly across the lines 51 and 59 so that the full voltage can be applied to the tube, and current can iiow during both half cycles of the impressed wave. Of course, under such circumstances it will be apparent no direct current potential is needed on the plates oi' the modulator tubes, and the energy which'is to be dissipated in the plate is reduced. v According to the present invention, the modulator tube 35 consists preferably of two triode sections having cathode elements 36 and 31, anode elements t and 39 and control electrodes d@ and M, in which the grid or control electrode sections should preferably be balanced to ground. As shown, the arrangement is so connected that the cathode 36 of the first tube section connects preferably directly across the conductors 59 and et, and the cathode 3l of the second tube section connects directly across the conductors 5l andV be, and the plate or anode of opposite sections of the tube connects to one of the same feeders or conductors as the opposite cathode section. The control or grid electrodes t@ and di are connected to opposite end terminals of a tank circuit t3 comprising the adjustable condenser t@ and the inductance t5, which is centertapped at the point t@ and the video signals which are supplied at the terminal B8 are fed through the coupling condenser te to the junction point it on the inductance element t5. The resistor 5@ connected between the inductance 35 and ground 5i is provided in order that the two sections of the tube 35 may be made selfbiasing.- Filament voltage for heating the cathcathode, the tube may be of the so-called beam ode elements stand 3i of the tube sections is provided at the terminal points 52, 52' and t, 53' by way of the conductors Si, 58 and 5S, 69 of the transmission line section preferably housed within shielding tubes.

The arrangement of the tube may preferably be such' that the tube sections are formed so there is slightly higher capacitance from the grid to the plate than from the grid to the filament, and because of this, some variances from the usual standard type of tube may be desired, but can readily be provided by a suitable selection of the tube geometry.

In the arrangement shown, the impedance between the grids or control electrodes tu and 6i of the tube sections of the tube 35 is materially increased by the addition of the resonant circuit 13 including the capacitance ed and indutcance d5, where the condenser id is preferably tuned to the carrier frequency and the inductance @it is tapped at its mid-point et for the purpose of introducing the video signals, where the tube sections are vso formed that thecapacitance from the grid to the plate is greater than from the grid to the lament. This causes the grid voltage to be reversed in such a way-as to lower the plate impedance of the modulator tubes, and also permits some current to flow during a portion of the cycle.

The provision of resistor 5d is such as to make the tube sections of the tube t5 self-biasing, as above noted, and the advantage of the arrangement then becomes apparent in that the modulator characteristic becomes extremely linear with respect to grid voltage within a range between i0% and 96% of the maximum possible antenna current without the modulator lines connected. With the introduction of the video signal at the point t@ and the feeding of the video signal through the condenser de to the mid-point of the inductance ed, it becomes apparent that the resulant inductive reactance of the inductance t5 to the video signals is essentially zero, and consequently it becomes further apparent that with the arrangement provided each of the tubes may draw a very heavy plate current, and since the tubes draw no grid current, in that the control electrodes never go positive relative to the type.

With the apparatus arranged as hereinabove described, the following may be assumed to be one explanation of the manner in which the operation takes place.

The impedance inverting line'-b is rst assumed to be disconnected from the junction points le and i5. In this case, the energy inverting line i and s is perfectly matched t'o the tank circuit 3 and the feeder line i@ and il to the antenna. Under .these circumstances, when the tank circuit is energized, no reiiections can occur on the impedance inverting line l and s,

and substantially all of the energy from the nal amplier i yflows into antenna 2i through the tank circuit, the impedance inverting lines and 9 and the feeder lines i@ and ii. If now a. short circuit is placed across the junction it and i5, the voltage applied to the antenna 2i through the feeder linesl iii and il is reduced substantially to zero and no energy ilows into the antenna. At the same time, the line l and i?, if properly adjusted, reects a very high impedance to the junction point t and B and, therefore, does not accept energy from the tank circuit 8.

Suppose now that the impedance inverting lines 51 and 5t are connected to junction point iii and i5 andthe impedance inverting line and d, and that the modulator i@ is carried to a nonconducting state. Under such conditions, the impedance at the modulating end of the line Si and 5 9 will obviously be very high and the line 57 and 5d will reflect a short circuit at junction point it and i5 which causes the antenna current to fall approximately to zero value. As the conductivity through the modulator device is increased so that the impedance decreases it ls apparent that the resistance or impedance as reiiected at the point it and It will be'increased until the modulator reaches a state of maximum conductivity which approachese a short circuit at the modulator end of the line and ed so that the impedance, when reflected at the point t! and i5, reaches a marximum, and, therefore, maximum current ows into the antenna through the feeder line it? and it.

It therefore becomes evident that the modulator device i@ should be capable of producing a relatively wide linear variation in conductivity. It likewise is important that the modulator should be capable oi producing a minimum of resistance across the impedance inverting line 5t and et with a maximum of modulating signal energy in the positive direction. a'

These results are accomplished by the present invention through the use ci' two separate triade sections oi the tube, conventionally represented byt, with provision made for applying the modulating signal energy to the control electrodes it and Si so that for the maximum sil in the positive direction the control electrodes are car ried to the nearest point in the positive direction which they are capable of attaining, so that the tube impedance then becomes a minimum, and, likewise, for a maximum signal in the negative t directionimpressed at terminal d@ the tube sections reach a cut-od' value and the tube impedance reaches a maximum.

While suitable values resistance and the like have been indicated for convenience oi reference on the drawing and particularly on Figure 1 thereof, it is oi course obvious that these values are given merely for illustrative purposes and are in no wayA to be considered as limit- 'iications may be made scope of the invention as may be made and used many and various modiin the disclosed system departing from the spirit or it is dened in the hereing for wide variances emciently. Accordingly,

without in any way matter-appended claims.

Having now described our invention, claim is:

1. A signal transmission system comprising a source of carrier frequency energy, a source oicntrol signals for modulating the said carrier frequency energy, and a signal radiating means,

what we fa tank circuit for' said carrier frequency energy source including a transmission line connected with the said source, an impedance inverting line having one end connected to receive energy from.

the said transmission line, a ieeder line connecting the. said impedance inverting line to the said sig. nal radiating means, a second impedance inverting line connected to the opposite end of the said tirst impedance inverting line, said second impedance inverting line being adapted to receive energy from the source of control signals, and a modulator means to supply said control signals to said second impedance inverting line. said modulator comprising first andsecond elements each including at least a cathod an anode and a control electrode and having the cathode of the nrst tube element and the anode of the second tube element connected to one side oi the said second impedance inverting line. and the cathode of the second tube element and the 'anode of the first tube element connected to the other side of the second impedance inverting line,

and means to supply signals from the source to 35' nected inductance and capacity elements connected between the control electrodes of said modulator tubes, and a' connection between the control signal source and the mid-point of said inductance element to supply signals to control thermionic tube the modulator tubes so that control signals cause the impedance across the junction of the first impedance inverting line and the feeder to the signal radiating means to vary between minimum and maximum for ka like range of variation in the control signal.

l4, The system claimed in claim l comprising, in addition, an oscillatory circuit including parallelly connected inductance and capacity elements connected to the control electrodes oi the said modulator tubes, means to supply signals from the source to the mid-point of the inductance to energize th e said control electrodes, and means to apply a self-biasing voltage to each of the said modulator tubes.

ROSCOE H. GEORGE.

Y RICHARD c. WEBB. 

